In radio beacon-based road toll systems, for example, DSRC (dedicated short-range communication) or WAVE (wireless access in a vehicle environment) standards, onboard units (OBUs) carried by the vehicles communicate with geographically distributed radio beacons, as soon as they pass the beacons. The radio communication generally serves to locate the vehicle on the radio coverage area of a radio beacon to charge for usage of locations or simply to transmit toll data generated by the OBU to the radio beacon.
It is often desirable to determine the distance at which the vehicle passes a radio beacon, for example, to penalise toll violations in the case of multi-lane roads. When multiple vehicles travelling next to one another on different lanes pass a radio beacon and a radio communications indicates a toll violation, e.g., a missed toll charge debit, an inadequate balance of a charge account, a defective or incorrectly adjusted OBU, or a lane-dependent charge rate or toll (multiple occupant lane), it is essential to know which of the vehicles travelling next to one another is responsible to be able to identify the vehicle in violation. The identification may be visually in situ or a photo of the vehicle in the road section of the beacon.
Various methods of determining the distance are known currently. One solution is to use multiple physically apart receiving antennas in the radio beacon to determine the positions of the OBUs in the radio receiving field from phase difference measurements between the OBU signals received by the individual antennas. Another solution is known from the U.S. Pat. No. 5,790,052 and is based on Doppler measurements of the different relative speeds of an OBU in relation to physically offset receiving antennas of a radio beacon to determine the ratio of the distances from the two receiving antennas from the ratio of the speed measured values. Finally, it would also be possible to use a separate radio beacon with a low radio coverage range for each lane. All these known solutions are expensive, because they are based on multiple receiving antennas.